Applicant has perfected and has been practicing a commercial process for several years for forming an alloy on the surface of a substrate. In this commercial process, an electrode is formed of the alloying material, for example, a carbide material. The electrode is connected to a pulse generating machine which creates repetitive spark discharges between the electrode and the substrate when the electrode is in engagement with the substrate. The electrode is rotated while in engagement with the substrate and caused to traverse the substrate to distribute the alloying material over the surface of the substrate. In this commercial process, it is believed that small amounts of the electrode material melt during each discharge and are transferred to the substrate. The discharge is also effective to either melt, soften, or otherwise create the conditions for alloying at the surface of the substrate so that the electrode material forms an alloy with the substrate material at the zone contiguous with the surface.
The above prior method, when correctly practiced, provides a very satisfactory surface alloy and has achieved a large degree of commercial success. However, the method is time consuming since there is only a small contact area between the surface of the substrate and the electrode. The small area of contact must traverse the entire surface in order to provide a continuous surface alloy over the entire surface. That operation is obviously time consuming. Additionally, this prior method is normally achieved by manual movement of the electrode across the surface area, and hence, the uniformity of the surface alloy and the continuity of the surface alloy over the entire surface depends upon the skill of the operator of the process. Furthermore, this prior method is not especially suitable for either automation or high levels of mass production.
In the prior patent art relating to a similar surface alloying method, a suggestion has been made that the surface be sprinkled with an alloying powder or covered with a slurry of alloying material prior to the surface alloying process. However, the powder or slurry method in the case of a rotating electrode/electric spark discharge method is not especially satisfactory because the electrode will tend to push the powder or slurry into piles as it is moved across the surface thereby making it extremely difficult to achieve a uniform distribution of the surface alloy.
The present invention provides a method and apparatus for forming a surface alloy which is capable of providing a uniformly alloyed surface, is particularly suitable for automation, and can substantially increase the speed of surface alloying over the prior method. In accordance with the present invention, a suitable alloying material is placed on the surface of a substrate and a beam of high intensity energy is directed at the alloying material for a predetermined short period of time. The duration of the predetermined short period of time and the intensity of the high intensity beam are interrelated and must be judiciously selected to achieve a suitable alloying action without damaging the substrate. By way of illustration, the substrate may be a machine tool part of critical dimensions and have a heat treatment which establishes critical strength or hardness properties of the tool. The surface must be alloyed without detrimentally altering the dimensions of the tool or detrimentally affecting the heat treatment of the substrate of the body of the tool. These conditions are met by restricting the period of time that the beam is directed upon a given area of the surface of a very short period of time so that the transfer of heat to the body of the substrate is minimized and heating is substantially restricted to the zone of the substrate which is contiguous the surface. The time is restricted by selecting a very high intensity beam of energy, such as a laser beam, which is capable of melting an alloying material in that very short period of time and is additionally capable of heating a zone of the substrate which is contiguous the surface during that short period of time sufficiently to provide an alloying action between the alloying material and the substrate material in the contiguous zone. If the intensity of the beam is too low, the period of time that the beam must be directed to a given area of the surface will be too long to avoid substantial heating of the substrate, and accordingly, the substrate may be detrimentally affected. The short period of time in combination with the related high intensity beam has an additional desirable effect. Particularly, when the heating is restricted to the contiguous zone the main body of the substrate acts as a heat sink into which the heat introduced into the zone contiguous the surface may be rapidly dissipated so that the alloyed surface zone is rapidly solidified. As a result, the treated parts may be handled soon after treatment without special precautions. The rapid cooling is also beneficial in that it provides the desired hardness of certain alloys. Additionally, surface deformation such as may be caused by "running" of the molten alloying material is avoided.
The preferred high intensity beam is a laser beam. Lasers of sufficiently high intensity to practice this method are presently in existence and known to the scientific community.
The preferred method of restricting the predetermined short period of time during which the beam is directed to a particular portion of the surface of the substrate is through scanning or traversing of the beam relative to the substrate. Either the beam or the part may be moved relative to the other so that the beam traverses or scans the surface area. Several means of scanning are disclosed in more detail hereinafter. On small parts, it may be desirable to restrict the time during which the beam is directed at the surface using a timing circuit rather than scanning the beam relative to the surface.
The scanning operation of the present invention makes this invention particularly suitable for automation, as will be apparent in view of the preferred embodiments of the apparatus of this invention. Automated scanning also assures that the surface alloy is created in a uniform manner and without the variations that can be expected if the alloying process is accomplished manually.